Shoot 'em ups (also known as shmups or STGs) are a subgenre of action games. There is no consensus as to which design elements compose a shoot 'em up; some restrict the definition to games featuring spacecraft and certain types of character movement, while others allow a broader definition including characters on foot and a variety of perspectives.

The genre's roots can be traced back to earlier shooting games, including target shooting electro-mechanical games of the mid-20th-century, but did not receive a video game release until Spacewar! (1962). The shoot 'em up genre was established by the hit arcade game Space Invaders, which popularised and set the general template for the genre in 1978, and has spawned many clones. The genre was then further developed by arcade hits such as Asteroids and Galaxian in 1979. Shoot 'em ups were popular throughout the 1980s to early 1990s, diversifying into a variety of subgenres such as scrolling shooters, run and gun games and rail shooters. In the mid-1990s, shoot 'em ups became a niche genre based on design conventions established in the 1980s, and increasingly catered to specialist enthusiasts, particularly in Japan. "Bullet hell" games are a subgenre of shooters that features overwhelming numbers of enemy projectiles, often in visually impressive formations.

A "shoot 'em up", also known as a "shmup" or "STG" (the common Japanese abbreviation for "shooting games"), is a game in which the protagonist combats a large number of enemies by shooting at them while dodging their fire. The controlling player must rely primarily on reaction times to succeed. Beyond this, critics differ on exactly which design elements constitute a shoot 'em up. Some restrict the genre to games featuring some kind of craft, using fixed or scrolling movement. Others widen the scope to include games featuring such protagonists as robots or humans on foot, as well as including games featuring "on-rails" (or "into the screen") and "run and gun" movement. Mark Wolf restricts the definition to games featuring multiple antagonists ("'em" being short for "them"), calling games featuring one-on-one shooting "combat games". Formerly, critics described any game where the primary design element was shooting as a "shoot 'em up", but later shoot 'em ups became a specific, inward-looking genre based on design conventions established in those shooting games of the 1980s.

Shoot 'em ups are a subgenre of action games. These games are usually viewed from a top-down or side-view perspective, and players must use ranged weapons to take action at a distance. The player's avatar is typically a vehicle or spacecraft under constant attack. Thus, the player's goal is to shoot as quickly as possible at anything that moves or threatens them to reach the end of the level, usually with a boss battle. In some games, the player's character can withstand some damage or a single hit will result in their destruction. The main skills required in shoot 'em ups are fast reactions and memorising enemy attack patterns. Some games feature overwhelming numbers of enemy projectiles and the player has to memorise their patterns to survive. These games belong to one of the fastest-paced video game genres.

Large numbers of enemy characters programmed to behave in an easily predictable manner are typically featured. These enemies may behave in a certain way dependent on their type, or attack in formations that the player can learn to predict. The basic gameplay tends to be straightforward with many varieties of weapons. Shoot 'em ups rarely have realistic physics. Characters can instantly change direction with no inertia, and projectiles move in a straight line at constant speeds. The player's character can collect "power-ups" which may afford the character's greater protection, an "extra life", health, shield, or upgraded weaponry. Different weapons are often suited to different enemies, but these games seldom keep track of ammunition. As such, players tend to fire indiscriminately, and their weapons only damage legitimate targets.

Shoot 'em ups are categorized by their design elements, particularly viewpoint and movement:

Fixed shooters restrict the player and enemies to a single screen, and the player primarily moves along a single axis, such as back and forth along the bottom of the screen. Examples include Space Invaders (1978), Galaxian (1979), Phoenix (1980), and Galaga (1981). In Pooyan (1982), the fixed axis of movement is vertical, along the right side of the screen. In Centipede (1981) and Gorf (1981), the player primarily moves left and right along the bottom, but several inches of vertical motion are also allowed within an invisible box.

Multidirectional shooters allow 360-degree movement where the protagonist may rotate and move in any direction such as Asteroids (1979) and Mad Planets (1983). Multidirectional shooters with one joystick for movement and one joystick for firing in any direction independent of movement are called twin-stick shooters. One of the first games to popularize twin-stick controls was Robotron: 2084 (1982).